As the operations of internal components of most electronic appliances (such as television, stereo, computer, etc) require the use of a direct current power supply, and thus a rectifier is needed to convert the alternate current power into a direct current power of various different voltages and maximize the functions of these electronic appliances. The rectifier can be divided into a linear rectifier and a switching rectifier according to different circuit structures, and the simple linear rectifier is consisted of a transformer, a diode, and a capacitor filter, and its advantages include simple circuit, high stability, quick responses to pause, high reliability, small ripple, and small electromagnetic interference. Since the linear rectifier adopts a low-frequency silicon steel transformer, therefore the linear rectifier has the shortcomings of large volume, heavy weight, low conversion efficiency (approximately 30% to 50%) and incapability of having a direct current input. To overcome the shortcomings of the linear rectifier, a switching rectifier is introduced. The switching rectifier has the advantages of high conversion efficiency, small power consumption for idle runs, light, and capability of having a direct current input, etc. Therefore, the current power supply market still uses the switching rectifier as the mainstream product. To cope with various different output powers, the following topologies for the rectifier circuits are developed for the switching rectifier, and these topologies include flyback, forward, full bridge, half bridge, and push-pull, etc.
Referring to FIG. 1 for the schematic view of a prior art half-bridge circuit, the half-bridge circuit comprises a main transformer T1, a power terminal B+of a direct current power supply connected to a preamble circuit, a pulse width modulation controller (PWMC), an isolation driven transformer T2, a DC isolation capacitor CBL, two input filter capacitors C10, C11, a current detection resistor RS, two current switches Q3, Q4, two output rectification diodes D1, D2 installed at the secondary winding of the main transformer T1, an energy storage inductor L1, an output filter capacitor C1, and a feedback control circuit Feedback-Control that feeds the signal of the output terminal OUTPUT back to the pulse width modulation controller PWMC. In the foregoing half-bridge circuit, the two current switches Q3, Q4 are N-channel field effect transistors, and the pulse width modulation controller PWMC produces a high/low pulse voltage level control signal to open/close the two current switches Q3, Q4 through an isolation driven transformer T2. The dots shown in all figures indicate the positive half cycle of the control signal corresponding to the polarity of each winding inductor voltage. In the positive half cycle, the control signal from the isolation driven transformer T2 maintains the current switch Q3 in the ON status and the current switch Q4 in the OFF status; in the negative half cycle, the control signal from the isolation driven transformer T2 maintains the current switch Q4 in the ON status and the current switch Q3 in the OFF status. For the operations of a half-bridge circuit, it is very important not to open the two current switches Q3, Q4 at the same time to avoid excessive current passing through and burning the current switches. To assure that this principle is followed, the pulse width modulation controller PWMC will close the two current switches Q3, Q4 simultaneously by the control signal within a short time between the positive half cycle and the negative half cycle, and this period of time is called the fly wheeling time. The outputted energy within the fly wheeling time is released by an energy storage inductor L1 and rectified by a diode D1, D2 circuit for the power supply. From the description above, the overall output rectifier circuit must pass through the rectifier diode D1 or D2 regardless of being in a positive/negative half cycle or a fly wheeling time, and the voltage of the rectifier diode will be dropped approximately to 0.4˜1.0V. Therefore, there will be a large energy loss for a large current output. The prior art full-bridge circuit is operated by the method similar to the foregoing half-bridge circuit, except that the primary winding uses four current switches, and thus the output power is twice as much as that of the half-bridge circuit. However, the arrangement of the secondary winding is identical to that of the half-bridge circuit.